To improve the sensitivity of a photodiode, it is desired to suppress leakage current when a reverse bias is applied to a pn junction of the photodiode. The leakage current serves as dark current that flows when light is not incident, leading to a reduction in sensitivity. In the case where a depletion layer formed in applying a reverse bias to the photodiode comes into contact with the interface between a semiconductor material such as silicon and an insulating material such as silicon oxide, leakage current is increased.
To form the ohmic contact between a conductive plug and a diffusion region on a surface side of the photodiode, the impurity concentration in a contact portion is set so as to be higher than that of a portion around the contact portion. For the case where the depletion layer comes into contact with the region with a high impurity concentration, leakage current due to crystal defects in the region with a high impurity concentration is increased.
A region where the interface between the semiconductor material and the insulating material is in contact with the depletion layer is localized around the contact portion, thereby suppressing the leakage current. Furthermore, the prevention of the extension of the depletion layer to the region with a high impurity concentration suppresses the leakage current.
A metal silicide film is often formed on surfaces of the source and the drain of a MOS transistor. Meanwhile, a metal silicide film is not formed on a contact portion of a photodiode in the related art. Different etching conditions in forming via holes in an interlayer insulation film are used for a portion where the metal silicide film is formed and a portion where the metal silicide film is not formed. Thus, two etching steps of forming the via holes are performed. In a second step of forming the via holes, the via holes that have been formed are filled with a mask material such as a resist. For example, residues of the mask material cause a reduction in yield.
A high-concentration region located at a contact portion of the diffusion region on the surface side of the photodiode is formed by ion implantation through a via hole passing through the interlayer insulation film. In this method, after the step of forming the interlayer insulation film, it is necessary to perform a heat-treatment step in order to activate impurity ions. The impurity ions in an impurity diffusion region that has been formed are diffused again during the heat treatment, thereby changing element properties. In the case of the change in element properties, circuit-design assets, such as various parameters for an existing circuit simulator and logic macros, are not used.